The development of a portable, easy to use instrument for the direct measurement of the alpha activity weighted particle size distributions in indoor environments composed primarily of radon decay progeny is proposed. These measurements will lead to a far greater precision in the estimation of the hazard due to environmental radon exposure than from simple radon level measurements alone. Our technique will allow the simultaneous measurement of the alpha particle energy spectrum of the size selected radio-aerosols which are sequentially deposited on wire screen filters in the detector over the particle size range 0.3 - 500 nm. Inhaled radon progeny, particularly those due to the "unattached" or ultrafine fraction, are estimated to be the dominant sources of background radiation that deliver significant doses to the tissues or cells of the respiratory tract. During Phase I of this project we constructed and tested a working laboratory version of this sampler to measure the ultrafine particle size distribution. We have demonstrated with this device that a relatively simple, low cost particle size sampler can be successfully constructed and that this monitor, using low cost solid state alpha particle detectors, is highly transportable and that the associated microprocessor controlled electronics can be reliably utilized to find the activity weighted particle size distributions of the polonium isotopes in a number of environments. During Phase II we will configure the technology into a relatively inexpensive system which will again be readily transportable for near real time measurement of activity weighted radon progeny particle size distributions in environments where complete characterization of the alpha activity is required. Phase III will involve commercialization of this technology through fabrication and sales of various stand-alone instruments specifically designed for a given application. We anticipate that large scale radon surveys will be performed quickly and at minimum cost with this easy-to-use and very transportable instrumentation.